Method for desulfurizing gasoline or diesel fuel for use in a fuel cell power plant

ABSTRACT

A fuel processing method is operable to remove substantially all of the sulfur present in an undiluted hydrocarbon fuel stock supply which is used to power a fuel cell power plant in a mobile environment, such as an automobile, bus, truck, boat, or the like; or in a stationary environment. The power plant hydrogen fuel source can be gasoline, diesel fuel, or other like fuels which contain relatively high levels of organic sulfur compounds such as mercaptans, sulfides, disulfides, thiophenes and the like. The undiluted hydrocarbon fuel supply is passed through a nickel reactant desulfurizer bed wherein essentially all of the sulfur in the organic sulfur compounds reacts with the nickel reactant, and is converted to nickel sulfide, while the now desulfurized hydrocarbon fuel supply continues through the remainder of the fuel processing system. The method involves adding hydrogen to the fuel stream prior to the desulfurizing step. The method can be used to desulfurize either a liquid or a gaseous fuel stream. The addition of hydrogen serves to extend the useful life of the nickel reactant. The hydrogen can be derived from source of pure hydrogen gas, a recycle gas stream, or can be derived from an electrolysis cell which breaks down water produced in the fuel cell into its hydrogen and oxygen components. The hydrogen when added to the fuel stock serves to prevent or minimize carbon formation on the nickel reactant bed, thereby extending the useful life of the reactant bed, since carbon deposits tend to block active sites in the reactant bed.

TECHNICAL FIELD

[0001] The present invention relates to a method for desulfurizinggasoline, diesel fuel or like hydrocarbon fuel streams so as to renderthe fuel more suitable for use in a mobile vehicular fuel cell powerplant assembly. More particularly, the desulfurizing method of thisinvention is operable to remove organic sulfur compounds found ingasoline to levels which will not poison the catalysts in the fuelprocessing section of the fuel cell power plant assembly. The method ofthis invention involves the use of a nickel reactant bed which has anextended useful life cycle due to the addition of hydrogen to the fuelstream in appropriate amounts.

BACKGROUND OF THE INVENTION

[0002] Gasoline, diesel fuel, and similar hydrocarbon fuels have notbeen useful as a process fuel source suitable for conversion to ahydrogen rich stream for small mobile fuel cell power plants due to theexistence of relatively high levels of naturally-occurring complexorganic sulfur compounds. Hydrogen generation in the presence of sulfurresults in a poisoning effect on all of the catalysts used in thehydrogen generation system in a fuel cell power plant. Conventional fuelprocessing systems used with stationary fuel cell power plants include athermal steam reformer, such as that described in U.S. Pat. No.5,516,344. In such a fuel processing system, sulfur is removed byconventional hydrodesulfurization techniques which typically rely on acertain level of recycle as a source of hydrogen for the process. Therecycle hydrogen combines with the organic sulfur compounds to formhydrogen sulfide within a catalytic bed. The hydrogen sulfide is thenremoved using a zinc oxide bed to form zinc sulfide. The generalhydrodesulfurization process is disclosed in detail in U.S. Pat. No.5,292,428. While this system is effective for use in large stationaryapplications, it does not readily lend itself to mobile transportationapplications because of system size, cost and complexity. Not only isthe hydrodesulfurization process more complicated because it is a twostep process, but to be effective in desulfurizing heavier fuelscontaining thiophenic sulfur compounds, it must operate at elevatedpressures, usually greater than about 150 psig.

[0003] Other fuel processing systems, such as a conventional autothermalreformer, which use a higher operating temperature than conventionalthermal steam reformers, can produce a hydrogen-rich gas in the presenceof the foresaid complex organic sulfur compounds without priordesulfurization. When using an autothermal reformer to process raw fuelswhich contain complex organic sulfur compounds, the result is a loss ofautothermal reformer catalyst effectiveness and the requirement ofreformer temperatures that are 200° F.-500° F. higher than are requiredwith a fuel having less than 0.05 ppm sulfur. Additionally, a decreasein useful catalyst life of the remainder of the fuel processing systemoccurs with the higher sulfur content fuels. The organic sulfurcompounds are converted to hydrogen sulfide as part of the reformingprocess. The hydrogen sulfide can then be removed using a solidabsorbent scrubber, such as an iron or zinc oxide bed to form iron orzinc sulfide. The aforesaid solid scrubber systems are limited, due tothermodynamic considerations, as to their ability to lower sulfurconcentrations to non-catalyst degrading levels in the fuel processingcomponents which are located downstream of the reformer, such as in theshift converter, or the like.

[0004] Alternatively, the hydrogen sulfide can be removed from the gasstream by passing the gas stream through a liquid scrubber, such assodium hydroxide, potassium hydroxide, or amines. Liquid scrubbers arelarge and heavy, and are therefore useful principally only in stationaryfuel cell power plants. From the aforesaid, it is apparent that currentmethods for dealing with the presence of complex organic sulfurcompounds in a raw fuel stream for use in a fuel cell power plantrequire increasing fuel processing system complexity, volume and weight,and are therefore not suitable for use in mobile transportation systems.

[0005] An article published in connection with the 21 st Annual PowerSources Conference proceedings of May 16-18, 1967, pages 21-26, entitled“Sulfur Removal for Hydrocarbon-Air Systems”, and authored by H. J.Setzer et al, relates to the use of fuel cell power plants for a widevariety of military applications. The article describes the use of highnickel content hydrogenation nickel reactant to remove sulfur from amilitary fuel called JP-4, which is a jet engine fuel, and is similar tokerosene, so as to render the fuel useful as a hydrogen source for afuel cell power plant. The systems described in the article operate atrelatively high temperatures in the range of 600° F. to 700° F. Thearticle also indicates that the system tested was unable to desulfurizethe raw fuel alone, without the addition of water or hydrogen, due toreactor carbon plugging. The carbon plugging occurred because thetendency for carbon formation greatly increases in the temperature rangebetween about 550° F. and about 750° F. A system operating in the 600°F. to 700° F. range would be very susceptible to carbon plugging, as wasfound to be the case in the system described in the article. Theaddition of either hydrogen or steam reduces the carbon formationtendency by supporting the formation of gaseous carbon compounds therebylimiting carbon deposits which cause the plugging problem.

[0006] Commonly owned co-pending U.S. patent application Ser. No.09/470,483, filed Dec. 22, 1999 describes a system and method fordesulfurizing gasoline and/or diesel fuel by passing the fuel through anickel reactant bed wherein a major portion of the sulfur in the fuel isconverted to nickel sulfide. The fuel stream contains an oxygenate suchas ethanol, methanol or MTBE which acts to extend the useful like of thenickel reactant bed by suppressing carbon formation on the reactant bed.The use of such oxygenates has been found to increase the capacity ofthe nickel reactant bed to convert sulfur in organic sulfur compounds inthe fuel to nickel sulfide by about five hundred percent. The operatingconditions of the system and method described in the above-noted patentapplication are suitable for use in mobile applications of fuel cellpower plants, such as those usable in powering vehicles. One problemincurred by using MTBE is that the MTBE itself decomposes to anunsaturated hydrocarbon so it adds to the total potential carbondeposited onto the nickel. Carbon formation tends to poison the reactantby blocking pores and active sites of the nickel reactant.

[0007] It would be highly desirable from an environmental standpoint tobe able to power electrically driven vehicles, such as an automobile,for example, by means of fuel cell-generated electricity; and to be ableto use a fuel such as gasoline, diesel fuel, naphtha, lighterhydrocarbon fuels such as butane, propane, natural gas, or like fuelstocks, as the fuel consumed by the vehicular fuel cell power plant inthe production of electricity. In order to provide such a vehicularpower source, the amount of sulfur in the processed fuel gas would haveto be reduced to and maintained at less than about 0.05 parts permillion.

[0008] The desulfurized processed fuel stream can be used to power afuel cell power plant in a mobile environment or as a fuel for aninternal combustion engine. The fuel being processed can be gasoline ordiesel fuel, or some other fuel which contains relatively high levels oforganic sulfur compounds such as thiophenes, mercaptans, sulfides,disulfides, and the like. The fuel stream is passed through a nickeldesulfurizer bed wherein essentially all of the sulfur in the organicsulfur compounds reacts with the nickel reactant and is converted tonickel sulfide leaving a desulfurized hydrocarbon fuel stream whichcontinues through the remainder of the fuel processing system.Previously filed U.S. patent applications Ser. No. 09/104,254, filedJun. 24, 1998; and Ser. No. 09/221,429, filed Dec. 28, 1998 describesystems for use in desulfurizing a gasoline or diesel fuel stream foruse in a mobile fuel cell vehicular power plant; and in an internalcombustion engine, respectively.

[0009] We have discovered that the capacity of a nickel reactant bed fordesulfurizing a gasoline or diesel fuel stream can be extended throughthe addition of hydrogen to the fuel stream in appropriate proportionswithout the need to include oxygenates in the fuel stream. The additionof hydrogen to the fuel stream essentially doubles the useful life ofthe nickel reactant bed over and above the procedure which utilizes theinclusion of oxygenates in the fuel stream.

DISCLOSURE OF THE INVENTION

[0010] This invention relates to an improved method for processing agasoline, diesel, or other hydrocarbon fuel stream over an extendedperiod of time, which method is operable to remove substantially all ofthe sulfur present in the fuel stream.

[0011] Gasoline, for example, is a hydrocarbon mixture of paraffins,napthenes, olefins and aromatics, whose olefinic content is between 1%and 15%, and aromatics between 20% and 40%, with total sulfur in therange of about 20 ppm to about 1,000 ppm. The national average for theUnited States is 350 ppm sulfur. The legally mandated average for theState of California is 30 ppm sulfur. As used in this application, thephrase “California Certified Gasoline” refers to a gasoline which hasbetween 30 and 40 ppm sulfur content. California Certified Gasoline isused by new car manufacturers to establish compliance with Californiaemissions certification requirements.

[0012] We have discovered that the addition of hydrogen (H₂) to thegasoline or diesel fuel stream extends the effective life of the nickelreactant sulfur-adsorption bed. The added hydrogen supresses carbondeposition on the nickel reactant bed, which carbon deposition wouldotherwise occupy and cover active sulfur-adsorption sites in the nickelbed, and could thereby shorten the effective life of the nickel reactantbed.

[0013] The effectiveness of a nickel adsorbent reactant to strip sulfurfrom organic sulfur compounds contained in gasoline or diesel fueldepends on the maintenance of as many active sulfur-adsorption sites inthe reactant bed for the longest possible time. In other words, thedesulfurization process depends on the amount of competitive adsorptionsites of the various sulfur-containing constituents of gasoline ordiesel fuel. From the adsorption theory, it is known that the relativeamount of adsorbate on an adsorbent surface depends primarily on theadsorption strength produced by attractive forces between the adsorbateand adsorbent molecules; secondarily on the concentration of theadsorbate in the gasoline, and temperature. Coverage of a reactantsurface by an adsorbate increases with increasing attractive forces;higher fuel concentration; and lower temperatures. Relative to gasoline,Somorjai (Introduction to Surface Chemistry and Catalysis, pp, 60-74)provides some relevant information on the adsorption of hydrocarbons ontransition metal surfaces, such as nickel. Saturated hydrocarbons onlyphysically adsorb onto the nickel reactant surface at temperatures whichare less than 100° F., therefore paraffins, and most likely naphthenes,won't compete with sulfur compounds for adsorption sites on the nickelreactant at temperatures above 250° F. and 300° F.

[0014] On the other hand, unsaturated hydrocarbons, such as aromaticsand olefins, adsorb largely irreversibly on transition metal surfaceseven at room temperature. When an unsaturated hydrocarbon such as anaromatic or an olefin adsorbs on a transition metal surface, and thesurface is heated, the adsorbed molecule rather than desorbing intact,decomposes to evolve hydrogen, leaving the surface covered by thepartially dehydrogenated fragment, i.,e., tar or coke precursors. Wehave discovered that, at 350° F., some unsaturated hydrocarbons aredehydrogenated, and the dehydrogenated tar fragments form multiplecarbon atom-to-nickel reactant surface bonds. This explains whyaromatics and olefins in gasoline or diesel fuel, in the absence of H₂in appropriate concentrations, will deactivate the nickel reactant fromadsorbing sulfur after a relatively short period of time.

[0015] In general, the adsorption strength of a compound depends on thedipole moment, or polarity, of the molecule. A higher dipole momentindicates that the compound is more polar and is more likely to adsorbon a reactant surface. Aromatics are an exception to this rule becausetheir molecular structure includes a π ring of electron forces thatproduces a cloud of induced attractive forces with adjacent surfaces.Based on the dipole moments of hydrocarbons, allowing for the π ring inaromatics, the order of adsorption strength (highest to lowest) is:nitrogenated hydrocarbons>oxygenatedhydrocarbons>aromatics>olefins>hydrocarbons containing sulfur>saturatedhydrocarbons. The presence of hydrogen in the gasoline or diesel fuelbeing scrubbed results in hydrogenation of the dehydrogenated byproductsof the desulfurized organic compounds which are adsorbed onto thereactant surface, which frees the byproducts from the nickel reactantadsorption sites. Thus, hydrogenation can reduce the adsorption ofdesulfurized aromatic and olefin byproducts on the nickel reactant bed.Although saturated hydrocarbons (paraffins and cycloparaffins) would notbe expected to be adsorbed on the desulfurization nickel reactant to asignificant extent, hydrogenation of olefins and aromatics will alsoprevent them from adsorbing onto the nickel reactant.

[0016] We have also discovered that the hydrogenated hydrocarbons do notinhibit the sulfur compounds from being adsorbed on the nickel reactantbecause they do not adsorb onto the nickel reactant surface attemperatures in the range of about 200° F. to about 500° F. The sulfurcompounds are quite polar and therefore contact and react with theactive nickel metal reactant sites.

[0017] Further non-essential but enabling information relating to thisinvention will become readily apparent to one skilled in the art fromthe following detailed description of a preferred embodiment of theinvention when taken in conjunction with the accompanying drawings inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic drawing of a system which is formed inaccordance with this invention for desulfurizing a gasoline or dieselfuel stream so that the latter may serve as a source of hydrogen forpowering an fuel cell power plant used to supply energy for operating avehicle;

[0019]FIG. 2 is a graph showing sulfur exit levels versus totaloperation time comparing recycle hydrogen from the selective oxidizerwith an MTBE additive but no hydrogen additive; and with neither MTBEnor hydrogen additive;

[0020]FIG. 3 is a graph comparing the catalyst loading with sulfur andthe sulfur exit level results noted when desulfurizing Californiapremium blend gasoline with MTBE and with hydrogen recycle from aselective oxidizer in the system; and

[0021]FIG. 4 is graph showing carbon deposition on a nickel reactant bedas a function of the length of the reactant bed.

SPECIFIC MODES FOR CARRYING OUT THE INVENTION

[0022] Referring now to the drawings, FIG. 1 is a schematic view of adesulfurizing system which can be used onboard a vehicle to providehydrogen to a fuel cell power plant that is used to produce electricityto operate the vehicle. The fuel being desulfurized can be gasoline ordiesel fuel, or some other fuel which is normally available to operatean internal combustion engine. It will be noted that all of thecomponents of the system are disposed onboard the vehicle in question.The system is denoted generally by the numeral 2 and includes a fuelsupply tank 4 and a line 6 which leads from the fuel tank 4 to a nickelreactant desulfurizer bed 8. The desulfurizer bed 8 may be heated tooperating temperatures by a battery 1 connected to the desulfurizer bed8 by cables 12. The desulfurizer bed 8 will preferably be run attemperatures which will vaporize the fuel stream entering thedesulfurizer bed 8. The desulfurized fuel passes through a line 14 to areformer 16, which is preferably an autothermal reformer. Thehydrogen-enriched reformed fuel passes through a line 18, through afirst heat exchanger 20 and thence through a line 22 into a second heatexchanger 24. The heat exchangers 20 and 24 serve to lower thetemperature of the reformed fuel stream while raising the temperature ofthe fuel, steam and air entering the reformer. The reformed fuel streamthen passes through a line 26 and thence through a water gas shiftconverter 27 before it enters a selective oxidizer 28 where CO in thefuel stream is oxidized to CO₂, before the H₂ enriched gas stream is fedto the fuel cell anode. The treated fuel stream exits the selectiveoxidizer 28 via line 32 and ultimately enters the fuel cell power plant55. A hydrogen recycle line 30 connects the selective oxidizer 28 andthe line 6 so that a controlled amount of hydrogen can be removed fromthe selective oxidizer 28 and recycled back into the fuel stream in theline 6. The hydrogen recycle line 30 could also be connected to thedesulfurizer bed 8 if so desired. The purpose of the recycle line 30 isto add a controlled amount of hydrogen (H₂) to the fuel stream as itenters the desulfurizer bed 8. The amount of hydrogen fed into the bed 8can be controlled by means of a pump or an ejector (not shown). Anejector is a device which is used to draw a secondary fluid into aprimary fluid stream with no moving parts, such as a Venturi tubeassembly.

[0023] The H₂ additive can also be derived from a source of H₂ 34 thatcan take the form of a hydrogen tank; a hydride bed; or an electrolysiscell which breaks down water from the fuel cell 55, or from some othersource, into H₂ and O₂. When water from the fuel cell is used, the waterwill be delivered to the H₂ source 34 by means of a line 36. H₂ from theH₂ source 34 is delivered to the line 6 via a line 38. As noted above,the addition of H₂ to the fuel stream results in hydrogenation ofadsorbed unsaturated hydrocarbons which will then desorb from the nickelreactant so as not to inhibit the sulfur compounds from being adsorbedon the nickel reactant.

[0024]FIG. 2 is a graph of the results of short term desulfurizer testruns which compares the effectiveness of hydrogen and MTBE asdesulfurizing gasoline with an gasoline which had no additives at all.It will be noted that the hydrogen additive resulted in a lowerdesulfurizer exit stream sulfur level for a longer time than the MTBE,and for a much longer time than when no additive was added to thegasoline. In the samples used in this test run, the additive-freegasoline and the hydrogen additive samples contained twenty one ppmsulfur at the desulfurizer inlet, and the sample to which MTBE was addedcontained twenty five ppm sulfur at the desulfurizer inlet. The amountof MTBE in the gasoline was 11% by weight, and the amount of hydrogenadded to the gasoline was 160 ml/min, which is equivalent to about 0.7%of the hydrogen exiting from the selective oxidizer. The temperature was350° F. and the space velocity was twenty six pounds of fuel per hourper pound of reactant.

[0025]FIG. 3 is a graph which compares the catalyst loading of sulfurand exit levels of sulfur in a vaporized gasoline stream of Californiaspecial blend gasoline, one of which gasoline streams included ahydrogen (H₂) additive, and the other of which included an MTBEadditive, but no H₂ additive. The solid line trace on the graphindicates the desulfurizer bed catalyst loading with sulfur and exitsulfur level of the fuel stream which was provided with an H₂ additive,and the broken line indicates the same data when the gasoline wasprovided with MTBE. The sulfur loading of the catalyst bed in eachinstance is also shown in FIG. 3. It will be noted that the sulfurlevels at the exit end of the desulfurizing bed 8 in ppm rise fasterwhen MTBE is used than when H₂ is used as an additive. It is also notedthat the ability of the nickel reactant to absorb sulfur increases whenH₂ is used as an additive, as compared to MTBE. The amount of hydrogenadded was 13 mole percent, the temperature of the tests was 375° F. andthe space velocity was two pounds of fuel per hour per pound ofreactant. The amount of hydrogen used in this test equaled about 1% ofthe hydrogen exiting the selective oxidizer, and was added to thegasoline stream by means of a simulated recyce stream from the selectiveoxidizer. The MTBE was present in an amount of 11% by weight.

[0026]FIG. 4 is a graph showing carbon deposition on the nickel reactantas a function of the length of the reactant bed shown in percentages ofthe total reactant bed length. The solid line indicates the extent ofcarbon deposition from a gasoline fuel which included MTBE but nohydrogen additive. The broken line indicates the extent of carbondeposition from a gasoline which included a hydrogen additive, but noMTBE. It can be seen that when 11% MTBE was added to the gasoline, morecarbon was deposited on the nickel reactant bed in two hundred eighteenhours than was deposited on the reactant bed when 13 mole % of hydrogenwas added to the gasoline after four hundred fifty hours. The additionof hydrogen to the gasoline being desulfurized enabled the nickelreactant surface to remain available for sulfur reaction for a muchlonger period of time, thus allowing a much higher sulfur loading on thereactant bed to be achieved.

[0027] We conclude that the presence of hydrogen in the gasolinemaintains the desulfurization activity of the nickel reactant bysignificantly suppressing the carbon content (coke deposits and stronglyadsorbed species), and by keeping the nickel reactant active sites cleanand available for desulfurization of the S-containing organic molecules.It will be readily appreciated that the addition of an effective amountof H₂ to a sulfur-containing fuel, will allow the sulfur to be removedfrom the fuel to the extent necessary for use of the fuel as a hydrogensource for a mobile fuel cell power plant without poisoning the fuelcell power plant reactant beds with sulfur. The sulfur compounds areremoved from the fuel by means of a nickel reactant bed through whichthe fuel flows prior to entering the fuel cell power plant's fuelprocessing section. The hydrogen serves to control carbon deposition onthe nickel reactant bed thereby extending the useful life of thereactant bed and enhancing the sulfur removal capabilities of the nickelreactant bed.

[0028] Since many changes and variations of the disclosed embodiment ofthe invention may be made without departing from the inventive concept,it is not intended to limit the invention otherwise than as required bythe appended claims.

What is claimed is:
 1. A method for desulfurizing a hydrocarbon fuelstream so as to convert the hydrocarbon fuel stream into a low sulfurcontent fuel, which low sulfur content fuel is suitable for use in afuel processing section in a fuel cell power plant, said methodcomprising the steps of: a) providing a nickel reactant desulfurizationstation which is operative to convert sulfur contained in organic sulfurcompounds in the fuel stream to nickel sulfide; b) introducing ahydrocarbon fuel stream which contains a molecular hydrogen (H2)additive into said nickel reactant desulfurization station; and c) saidH2 additive being present in said fuel stream in an amount which iseffective to suppress carbon deposition on said nickel reactant andprovide an effluent fuel stream at an exit end of said nickel reactantstation which effluent fuel stream contains no more than about 0.05 ppmsulfur.
 2. The method of claim 1 wherein the H₂ additive is derived froma container of H2 in the fuel processing section of the fuel cell powerplant.
 3. The method of claim 1 wherein said H₂ additive is derived fromrecycled reformed fuel gas from a selective oxidzer in the fuelprocessing section of the fuel cell power plant.
 4. The method of claim1 wherein said H₂ additive is derived from an electrolysis cell in thefuel processing section of the fuel cell power plant which convertswater to H₂ and O₂.
 5. A method for desulfurizing a gasoline fuel streamso as to convert the gasoline fuel stream into a low sulfur contentfuel, which low sulfur content fuel is suitable for use in a fuelprocessing section in a fuel cell power plant, said method comprisingthe steps of: a) providing a nickel reactant desulfurization stationwhich is operative to convert sulfur contained in organic sulfurcompounds contained in the fuel stream to nickel sulfide; b) introducinga gasoline fuel stream which contains a hydrogen (H₂) additive into saidnickel reactant desulfurization station; and c) said H2 additive beingpresent in said gasoline fuel stream in an amount which is effective toprovide an effluent gasoline fuel stream at an exit end of said nickelreactant station which effluent gasoline fuel stream contains no morethan about 0.05 ppm sulfur.
 6. A system for desulfurizing a gasoline ordiesel fuel stream so as to convert the gasoline fuel stream into a lowsulfur content fuel, which low sulfur content fuel is suitable for usein a fuel processing section in a fuel cell power plant, said systemcomprising: a) a nickel reactant desulfurization station which isoperative to convert sulfur contained in organic sulfur compoundscontained in the fuel stream to nickel sulfide; b) means for introducinga gasoline or diesel fuel stream into said nickel reactantdesulfurization station; and c) a supply of a hydrogen (H₂) additive andmeans connecting said H₂ additive supply to said fuel stream, said H₂additive being present in said fuel stream in an amount which iseffective to provide an effluent fuel stream at an exit end of saidnickel reactant station which effluent fuel stream contains no more thanabout 0.05 ppm sulfur.
 7. The system of claim 6 wherein said supply ofH₂ additive is derived from recycled gas from a fuel cell power plantselective oxidizer.
 8. The system of claim 6 wherein said supply of H₂additive is derived from a container of H₂.
 9. The system of claim 6wherein said supply of H₂ additive is derived from a hydride bed. 10.The system of claim 6 wherein said supply of H₂ additive is derived froma water electrolysis cell.